C, Y = map(int, input().split())
if C * 100 > Y:
    exit(print("can't exchange"))

N = 10
cnt500 = Y // 500
Y %= 500
cnt100 = Y // 100

if cnt100 >= C:
    exit(print("no exchange"))

ok = cnt500; ng = 0
while abs(ok - ng) > 1:
    mid = (ok + ng) // 2
    if mid * 5 + cnt100 >= C:
        ok = mid
    else:
        ng = mid

print(ok * 5 + cnt100)